1. Field of the Invention
The present invention relates to improvements in potentiating and operating metal oxide sensors for the detection of a variety of gases and vapors. In particular, the present invention relates to the use of metal oxide sensors having a lower operational power requirement and an automatic regulation of the sensor's surface temperature.
2. Description of the Related Art
The use of metal oxide sensors for the detection of a variety of gases and vapors is well known. These sensors are rugged, very sensitive and relatively inexpensive to manufacture. They respond to the presence of the target gas by changing the conductivity of the material over a large range of values which allows for the use of simple electronic circuits to produce a useful output. Some form of Tin Oxide is often used in this capacity.
At temperatures of several hundred degrees Celsius, the metal oxide material combines with the Oxygen molecules in the air to produce a layer of negatively charged ions adsorbed to the surface of the material. The conduction of electricity in the metal oxide depends on the conductivity characteristics across the grain boundaries which make up the structure of the material. The layer of negatively charged ions on the surface of the material impedes the flow of negative charges across the boundaries making the material a poor conductor in the presence of clean air.
In the presence of reducing gases, however, the negative ions on the surface of the sensor become neutralized to an extent which depends on the concentration of the gas. This lowers the potential barrier across the grain boundaries and consequently, the conductivity of the material increases in proportion to the concentration of the gas.
One common issue that arises with the use of metal oxide sensors in a mobile setting stems from the amount power required to bring the sensor to its operating temperature. Because the temperature of the metal oxide sensing material must be brought to a relatively high temperature in order to operate, a typical sensor requires a substantial amount of electrical power in order to properly operate. At the present time, it is common for a metal oxide sensing material to require close to 1 Watt of power to elevate the material at a suitable operating temperature. For portable gas detectors using metal oxide sensors, however, this much power consumption results in poor battery life. One way to reduce this power requirement would be to use a sensor with a mass small enough to have relatively short thermal time constant.
In recent months, the introduction of new manufacturing techniques has led to the introduction of ultra miniature metal oxide sensors which require very little power to bring the surface temperature to the required high level for optimum operation. The miniaturization has allowed for the development of alternate techniques and arrangements that would reduce the power required to operate metal oxide sensors.
The introduction of such ultra miniature metal oxide sensors has, however, exacerbated certain existing challenges pertaining to the operation of metal oxide sensors, To achieve maximum efficiency, the atmosphere to be sampled must be drawn over the surface of the metal oxide sensor. The temperature of the metal oxide sensor is affected by the air current created when the atmosphere is drawn over its surface, or when the probe containing the sensor is moved rapidly through the air. For example, in the most basic operation of a metal oxide sensor, the atmosphere being sampled is often drawn over the sensor, which is typically done with the aid of a pumping mechanism. The air current created as a result is not necessarily constant and its variations cause the sensor's temperature to fluctuate. Also, modem portable leak detector usage often times demand that the technician sweep the sampling probe of the instrument over the suspected area, thereby producing variable air currents over the sensor and changing its temperature. If such an occurrence caused a metal oxide sensor to be unable to maintain a suitable operating temperature, the accuracy of its measurements would be adversely affected. The use of ultra miniature metal oxide sensors exacerbates this problem because a sensor with a smaller mass would likely have even more difficulty maintaining its operating temperature while in use.
U.S. Pat. No. 7,820,949 discloses a temperature control method that protects a sensor from damage and eliminates interference from ambient conditions such as condensation. The methods disclosed do not seek to maintain the temperature of the sensor at an optimum operating point or to do so while minimizing the power consumed.
U.S. Pat. No. 7,631,537 discloses a gas sensing apparatus that measures the thermal conductivity of a gas in an atmosphere containing moisture. This apparatus alternatively switches power from a heating element to a reference resistor to allow for the measurement of the resistance of the heating element. When the thermal conductivity of the gas surrounding the heating element changes so does the temperature of the heating element and its resistance. Notably, the apparatus disclosed does not keep the temperature of the heating element constant to provide an optimum condition for the sensing element to react to the presence of the gas being detected.
U.S. Pat. No. 7,350,396 teaches a system and method for identify the various gases in a mixture by varying the temperature of a metal oxide sensor by sending pulses of varying voltage amplitude to the heater. The effect of this is to heat the sensing element over a wide range of temperatures and utilize the response of the sensing element at different temperatures.
U.S. Pat. No. 6,644,098 discloses a method, system and apparatus for sensing the presence of at least one predetermined gas. While this reference seeks to regulate the temperature of the heater, it does not teach doing so using simple switching means and in a power conserving way.
U.S. Pat. No. 5,526,280 teaches a system and method for using a gas detecting device that first burns off oxides formed during periods of inactivity and then to brings the sensing element to the normal operating temperature. The sensor disclosed, however, could supply inaccurate readings if placed in air currents such as those produced when the sensor is used in conjunction with a pump to draw a sample of the gas because no attempt is made to regulate the temperature of the heater during operating.
U.S. Patent Application 2010/0089122 discloses a gas sensor system that seeks to conserve power by operating the gas sensor heater at a reduced temperature until increasing levels of the gas to be sensed are detected. At that point, the heater temperature is increased to improve the sensitivity of the sensing element at the higher contamination levels. There is no teaching to keep the temperature of the heating element in a gas sensor constant and the temperature of the heater is not regulated since the heater is always powered by fixed amplitude pulses of fixed duration and no feed back mechanism is disclosed to control the temperature of the heater. Power is simply also conserved by operating the sensor for short periods of time followed by long periods of inactivity until the concentration of the contaminant is sensed to increase.
U.S. Patent Publication 2010/0122568 teaches a sensor system in which the temperature of a gas sensor is measured and it is used to determine the characteristics of the drive method applied to the gas sensor. The heater control is used to maintain the temperature of the gas sensor and its environment within a predetermined range.
JP9043184A and JP9138209A each teach gas sensing methods and apparatuses. While these publications disclose pulsing the drive voltage to the heater and the sensing element, neither deals with temperature reducing power requirements while regulating of the heater in a metal oxide sensor in the presence of air currents and other ambient conditions.
What is needed is a gas sensing method and apparatus that utilizes metal oxide sensors that require less power to bring their temperature to the high level required for optimum operation and while providing enhanced reliability by maintaining them at the optimum operating temperature by automatically regulating the temperature of the heater in the presence of air currents and other ambient conditions.